This invention relates to medical diagnostic ultrasound imaging, and in particular to methods and systems for distinguishing contrast agent from surrounding tissue.
Contrast agent imaging is an important medical diagnostic ultrasound imaging mode. One limitation in many contrast imaging systems is the difficulty of distinguishing echo signals from contrast agents from echo signals from surrounding tissue. This is because both contrast agents and tissue generate nonlinear return signals at frequencies other than the frequency of the insonifying signals. It is well recognized that nonlinear signals from tissue are generated by nonlinear propagation of the insonifying ultrasound wave.
Prior-art harmonic imaging methods that exploit non-linear behaviour of contrast agent include B-mode harmonic imaging, B-mode harmonic pulse inversion imaging, harmonic power Doppler imaging, and color harmonic pulse inversion imaging.
In B-mode harmonic imaging, the signal is transmitted at a fundamental frequency f, and the receive signal is filtered to emphasize frequency components near the second harmonic, 2f. Contrast agents are known to have a stronger second harmonic response than tissue, and for this reason the receive signal from contrast agent is enhanced over that from tissue. This method is currently used by most major manufacturers of ultrasound imaging equipment. Specific examples are described in Mine U.S. Pat. No. 5,724,976, Uhlendorf U.S. Pat. No. 5,410,516, Schutt U.S. Pat. No. 5,733,527, and "Simulated Capillary Blood Measurement Using a Nonlinear Ultrasonic Contrast Agent," Schrope et al.; Ultrasonic Imaging, Vol. 14, pp. 134-158, 1992.
In B-mode harmonic pulse inversion imaging, two pulses are transmitted along the same ultrasound line, where one pulse is shifted by 180.degree. with respect to the other. Receive signals from the two pulses are then summed, and the resultant signal is displayed. The pulse inversion technique cancels stationary fundamental frequency signals and retains second harmonic signals as well as some nonstationary fundamental signals. Examples of such methods include Chapman U.S. Pat. No. 5,632,277 and Hwang U.S. Pat. No. 5,706,819.
In harmonic power Doppler imaging, multiple pulses are transmitted along the same ultrasound line, and the receive signal is filtered about the second harmonic frequency. The resultant signals are then filtered with a high-pass filter to remove stationary signals. Examples of such methods are described in Averkiou U.S. Pat. No. 5,833,613, Johnson U.S. Pat. No. 5,456,257, and "Harmonic Power Mode Doppler Using Microbubble Contrast Agents: An Improved Method for Small Vessel Flow Imaging," Burns, et al., 1994 IEEE Ultrasonic Symposium, pp. 1547-1550, 1994.
In color harmonic pulse inversion imaging, the approach used is similar to that used in harmonic power Doppler imaging described above, but alternate pulses are shifted in phase by 180.degree.. Such methods are described in "Pulse Inversion Doppler: A New Method for Detecting Nonlinear Echoes from Microbubble Contrast Agents," Simpson and Burns, 1997 IEEE Ultrasonic Symposium, 1997.
Though these techniques succeed in presenting contrast agent enhanced images, they do not entirely meet the needs of clinicians in the field. B-mode harmonic imaging does not differentiate between second harmonic signals generated by nonlinear propagation through tissue and second harmonic signals generated by contrast agents. Pulse inversion imaging further enhances second harmonic signals, but it still does not differentiate between second harmonic signals generated by nonlinear propagation through tissue and second harmonic signals generated by contrast agents. Harmonic power Doppler imaging attempts to differentiate contrast agent from tissue by looking for a loss of correlation between successive pulses due to agent destruction, agent motion or other methods. However, tissue motion will also result in a loss of correlation and may appear as a displayed signal. Tissue motion is reduced currently by a combination of increasing the pulse repetition frequency and/or the use of more aggressive clutter filters. However, increasing the pulse repetition frequency may reduce the signal from destroyed contrast agent, and more aggressive clutter filters may filter out contrast signal as well as tissue signal. These problems and drawbacks apply also to color harmonic pulse inversion techniques.
It would be advantageous to improve the specificity for imaging contrast agent as opposed to tissue. Increased specificity would allow contrast agent to be used for sensitive qualitative and quantitative measurements of blood flow in tissue.